Duplicate frozen string value when force encoding in make_json

[soedar]

When using the gelf formatter, like so:

<match **>
  @type stdout
  format gelf
</match>

We run encounter the following error:

#0 Error trying to serialize [original string]: can't modify frozen String

It appears that in the make_json function, the string values in gelfentry could potentially be a frozen string. This PR duplicates the string before calling force_encoding.